GHC tries to do so, but sometimes gets it wrong. See the -fwarn-incomplete-patterns flag. We'd appreciate it if
someone could
overhaul this code - it's been on the wish list for a long time.
Indeed, I always try to avoid all warnings in my sources by using the flag "-Wall", because I consider this to be good programming style. (In particular warnings about unused and shadowed variables prevented a lot of errors.) However some warnings are difficult to avoid. So how difficult would it be to implement non-exhaustive pattern warnings for nested patterns? data Color = Red | Green | Blue f :: Color -> String f x = case x of Red -> "r" _ -> " " ++ case x of Green -> "g" Blue -> "b" Warning: Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: Red "Red" in the second case is unreachable (and this should also be worth a warning.) Christian